1. Field of the Invention
The present invention relates to a phase plate and method of fabricating it.
2. Description of Related Art
In recent years, transmission electron microscopes capable of phase difference imaging have been regarded as promising instruments because they enable high-contrast imaging of samples containing large amounts of light elements such as biological samples and macromolecular samples. For example, in JP-A-200-273866, there is proposed a transmission electron microscope capable of phase difference imaging by incorporating a phase plate in the back focal plane of an objective lens. Generally, a phase plate consists of a thin amorphous film having through-holes. The phase plate is placed in the back focal plane of the objective lens such that transmitted waves pass through the through-holes, while scattered waves pass through the thin film. Therefore, the scattered waves passed through the through-holes can be shifted in phase by π/2 with respect to the transmitted waves passed through the through-holes. A transmission electron microscope equipped with a phase plate can provide a phase difference image of a cosine-type phase contrast transfer function by making transmitted waves and scattered waves shifted in phase by π/2 interfere with each other by the phase plate.
Conventionally, a phase plate has been manufactured by forming a thin film on a substrate made of a glass slide or mica having a cleaved face, peeling off the thin film at a water surface, transferring the thin film onto a phase plate support made of a molybdenum grid, and forming through-holes in the thin film by a focused ion beam (FIB) machine. In this way, it is not easy to carry out the process sequence for phase plates. Especially, the step of transferring the thin film onto the phase plate support and the step of forming the through-holes by the FIB machine require skilled techniques. This manufacturing method needs a long work time to manufacture the phase plate. Also, the manufacturing yield is low.
Accordingly, there is a demand for a method of manufacturing phase plates with simple process steps.